The process of roll forming relatively small radius bends in most sheet metal materials (steel, aluminum, copper, zinc, etc.) is fairly simple. However, the process of roll forming relatively large radius bends is much more difficult. Sheet metal materials for which roll forming is carried out to form radius curves typically ranges from 0.01 inch to 0.10 inch in thickness.
Metallic or sheet metal materials have both elastic and ductile properties. When sheet metal is bent or formed below its elastic limit, it will spring back to its original shape. The elastic limit must be exceeded to effect permanent deformation, i.e. a bend. In bending sheet metal to a selected angle/radius, a certain amount of spring-back or return to the original shape will take place. Although a portion of the metal in a bend is permanently deformed, portions of any bend will remain below the elastic limit and cause spring-back. In order to achieve a finished bend angle/radius, the sheet metal must be over bent. A technique for overbending is disclosed in U.S. Pat. No. 5,551,272. This might include bending through a larger angle, bending with a smaller inside bend radius, or both.
As the inside bend radius becomes smaller, the amount of spring-back is reduced. In small radius bending situations, sheet metal may be bent to its finished angle/radius, and little spring-back will be experienced. This simplifies the roll forming process in that a profile may be bent to its finished form, with few provisions included for over bending. Small radius bends require little over bending. In addition, small radius bends tend to require fewer variations in over bending as sheet material physical properties vary.
As the inside bend radius becomes larger, the amount of spring-back increases dramatically. In large radius bends, sheet metal must be severely over bent to achieve a finished angle/radius. This complicates the roll forming process in that significant provisions must be made to contend with over bending, which can becomes somewhat unpredictable as inside bend radii become larger and sheet metal physical properties vary.
As defined herein a small radius bend is a bend whose inside bend radius measures between zero to five times the material thickness. A large radius bend is above five times the material thickness. For a typical sheet metal material having a 0.03 inch thickness then a large radius bend is 0.15 inch and above.